The present invention relates generally to a brake valve device for use in industrial vehicles, and more particularly to a spool-out type booster brake valve device in which a valve spool is upwardly moved in accordance with the depression of a brake pedal, thereby causing high pressure working fluid to be introduced into a booster chamber to forcibly extend a master piston so that a brake fluid existing in a master chamber can be discharged under pressure into a brake actuator.
Generally, industrial vehicles such as forklift trucks are equipped with a pump for discharging high pressure working fluid. The working fluid discharged from the pump is supplied to a steering system, a brake system, and a variety of working implements so that it can be used as a power source for desired tasks to be conducted. The brake system typically includes a pedal adapted to be depressed by the foot of a user, a brake actuator adapted to apply a brake force to the wheels of a vehicle, and a brake valve device adapted to feed a brake fluid under pressure to the brake actuator in response to the depression of the pedal, thereby causing the brake actuator to conduct its braking function. The brake valve device may be either of a booster type in which the hydraulic force of the working fluid is used to force the brake fluid under pressure to the brake actuator, or a non-booster type in which only the depression force of the pedal is relied upon to direct the brake fluid to the brake actuator. The brake valve device may also be of a spool-in type in which a spool incorporated in the brake valve device is inwardly moved in accordance with the depression of the pedal, or a spool-out type in which the spool is outwardly moved in accordance with the depression of the pedal.
Referring to FIG. 1, a conventional brake valve device is illustrated which is of a spool-in booster type. As shown in FIG. 1, this brake valve device includes a valve body 10 which is defined with a booster chamber 12 and a master chamber 14. The booster chamber 12 is selectively connected to a pump 18 through a working fluid inlet 16 or to a reservoir tank 22 through a working fluid outlet 20. A valve spool 24 is slidably arranged in the valve body 10 in such a fashion that it can be shifted between an idle position and an operating position thereof. The valve spool 24 is normally biased toward the idle position by a return spring 24a. When the valve spool 24 is at its idle position, as shown in FIG. 1, the booster chamber 12 is shut off from the pump 18 while communicating with the reservoir tank 22. In this state, accordingly, no pressure is applied to the booster chamber 12. As the valve spool 24 is moved from its idle position to its operating position in accordance with the depression of a brake pedal 26, the pump 18 is connected to the booster chamber 12 so that high pressure working fluid can be supplied to the booster chamber 12. A pressure reducing valve 28 is arranged between the working fluid inlet 16 of the valve body 10 and the pump 18. The pressure reducing valve 28 serves to reduce the pressure of the working fluid supplied to the booster chamber 12 to a predetermined level or less.
In the meantime, the master chamber 14 of the valve body 10 is connected to a brake actuator 32 via a backflow control valve 30 while being connected to the valve spool 24 via a pilot line 34. The master chamber 14 also communicates with a brake fluid reservoir tank 36 to receive a brake fluid from the reservoir tank 36. A booster piston 38 and a master piston 40 are slidably received in the booster chamber 12 and the master chamber 14, respectively. These pistons 38 and 40 are connected to each other by means of a connecting rod 42 so that they can move as a unit. The booster piston 38 and the master piston 40 are always biased toward their retracted positions by return springs 38a and 40a, respectively. When the driver of a vehicle, to which the brake valve device is applied, depresses the brake pedal 26 to shift the valve spool 24 from the idle position to the operating position, the pump 18 is rendered to communicate with the booster chamber 12 to feed high pressure working fluid to the booster chamber 12. As a result, both the booster piston 38 and the master piston 40 are extended against the return springs 38a and 40a by a distance proportional to the displacement of the valve spool 24, thus causing the brake fluid in the master chamber 14 to be fed under pressure to the brake actuator 32 which in turn applies brake force to vehicle wheels.
On the contrary, when the depression force 26 is removed from the brake pedal, the valve spool 24 returns to its idle position by virtue of the biasing force of the return spring 24a. This allows the working fluid in the booster chamber 12 to be discharged to the reservoir tank 22. At this time, the booster piston 38 and the master piston 40 are moved to their retracted positions by virtue of the urging force of the return springs 38a and 40a, respectively, whereby the brake fluid is returned back to the master chamber 14 from the brake actuator 32 to make the latter inoperative.
In the case of the above mentioned spool-in type booster brake valve device, it is necessary to use an independent brake fluid reservoir tank for storage of brake fluid to be supplemented in the master chamber, as well as a pressure reducing valve for decreasing the pressure of the working fluid to be supplied to the booster chamber. Use of such elements tends to make the brake valve device complicated in structure and costly to manufacture. Moreover, it is unavoidable to replace overall link mechanism with a new one when an attempt is made to retrofit the existing spool-out type brake valve devices through the use of the spool-in type valve device referred to supra.
With the above-identified problems in mind, it is an object of the invention to provide a brake valve device for industrial vehicles capable of allowing pump-discharged working fluid to be used as a brake fluid, thereby eliminating the need to use a separate brake fluid fill-up unit.
Another object of the invention is to provide a brake valve device for industrial vehicles that permits pump-discharged working fluid to be supplied to a booster chamber without any pressure reduction thereof, thereby avoiding the use of a pressure reducing valve.
A further object of the invention is to provide a brake valve device for industrial vehicles which can be advantageously used in retrofitting existing spool-out type booster brake devices.
In accordance with the present invention, there is provided a brake valve device for use in industrial vehicles equipped with a fluid pump, a reservoir tank and a brake actuator, comprising: a valve body having an inlet port, a drain port, a brake port, a booster chamber communicatable with the pump via the inlet port and a master chamber communicatable with the brake actuator via the brake port, the master chamber selectively connectable to the pump to receive working fluid therefrom; a valve spool slidably fitted in the valve body through the booster chamber and the master chamber for extending and retracting movement with respect to the valve body, the valve spool shiftable between an idle position and an operating position; and slider means axially displaceably fitted to and movable together with the valve spool for, in response to the shifting movement of the valve spool into the operating position, decreasing the volume of the master chamber so that the fluid present in the master chamber can be extruded into the brake actuator via the brake port.